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Subject: Re: [PATCH v2 09/10] clocksource/drivers/hyper-v: Set clocksource
 rating based on Hyper-V feature
To:     Michael Kelley <mikelley@microsoft.com>, sthemmin@microsoft.com,
        kys@microsoft.com, wei.liu@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, arnd@arndb.de,
        linux-hyperv@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-arch@vger.kernel.org
References: <1614561332-2523-1-git-send-email-mikelley@microsoft.com>
 <1614561332-2523-10-git-send-email-mikelley@microsoft.com>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <e6e2f860-d109-25c7-5892-ef9b06daaa7d@linaro.org>
Date:   Mon, 1 Mar 2021 16:42:04 +0100
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On 01/03/2021 02:15, Michael Kelley wrote:
> On x86/x64, the TSC clocksource is available in a Hyper-V VM only if
> Hyper-V provides the TSC_INVARIANT flag. The rating on the Hyper-V
> Reference TSC page clocksource is currently set so that it will not
> override the TSC clocksource in this case.  Alternatively, if the TSC
> clocksource is not available, then the Hyper-V clocksource is used.
> 
> But on ARM64, the Hyper-V Reference TSC page clocksource should
> override the ARM arch counter, since the Hyper-V clocksource provides
> scaling and offsetting during live migrations that is not provided
> for the ARM arch counter.
> 
> To get the needed behavior for both x86/x64 and ARM64, tweak the
> logic by defaulting the Hyper-V Reference TSC page clocksource
> rating to a large value that will always override.  If the Hyper-V
> TSC_INVARIANT flag is set, then reduce the rating so that it will not
> override the TSC.
> 
> While the logic for getting there is slightly different, the net
> result in the normal cases is no functional change.
> 
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>

[ ... ]

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